Corona discharge reactor

Abstract: A corona discharge reactor which has a central electrode in the form of a metal rod with radial projections and a cylindrical outer electrode surrounding the inner electrode.

Description: The present invention relates to reactors for the processing of materials in the gaseous phase by means of corona discharges. Considerable effort is being expended on the development of techniques for carrying out processes in the gaseous phase, using species which have been activated by corona discharges. Corona discharges occur in gaseous media when the localised electric field in the neighbourhood of a body exceeds the electrical breakdown voltage of the gaseous medium. Existing corona discharge reactors consist of a chamber having an inlet and an outlet for a gaseous medium, an axial wire inner electrode and a cylindrical outer electrode surrounding the inner electrode. The electrical discharge within such reactors consists of streamers extending from the inner electrode towards the outer electrode. For the effective processing of a gaseous medium passing through the reactor, it is necessary to produce as many corona streamers as possible because any space which does not contain such streamers is dead space as far as the processing of the gaseous medium is concerned. Unfortunately, the rapid fall-off in the electric field in a central wire electrode type of corona discharge reactor means that the corona discharge streamers propagate only a short distance from the central wire. This phenomenon restricts the diameter of a practicable reactor to a few centimetres. It is not practicable to compensate for this by increasing the length of the reactor because of the lack of mechanical strength of the inner electrode. Thus existing corona discharge reactors have diameters of a few centimetres and lengths of a few tens of centimetres. As a result, to process reasonable volumes of gas, high flow rates are required, which in turn tend to increase the mechanical instability of the central electrode due to aerodynamic effects as well as leading to short residence times in the reactor chamber of the medium to be processed. Hence, existing corona discharge reactors are limited inherently in their effectiveness. US patent 5,061,462 discloses an apparatus for producing a corona discharge, specifically for the purification of internal combustion exhaust gases, in which a streamer type corona discharge is established in a reactor chamber between a first electrode, which may be in the form of a rod with a screw thread formed in its surface to provide an extended edge to facilitate the formation of a corona discharge and a second electrode which has an insulating surface (the reactor chamber wall or an oil film on the reactor chamber wall), facing the first electrode. It is claimed that the presence of the insulating layer on the second electrode stabilises the corona discharge between the electrodes. In practice, this is not necessarily so, and the presence of the insulator with its relatively low thermal conductivity, can lead to localised hot spots as well as reducing the electrical efficiency of the discharge within the reactor chamber. French patent 2,121,682 also discloses an apparatus for producing a corona discharge for use in the purification of internal combustion exhaust gases in which there is a central rod electrode having radial projections thereon and the wall of a reactor chamber surrounding the central electrode is used as an outer electrode. The arrangement has an inherently high capacitance which severely reduces the efficiency of the production of a corona discharge when the apparatus is used in a pulsed or AC mode. It is an object of the present invention to provide an improved form of corona discharge reactor for use in the processing of gaseous media by means of electrically activated species. According to the present invention there is provided a corona discharge reactor for use in the processing of materials, comprising a reactor chamber through which a gaseous medium to be processed can be caused to flow, a first cylindrical electrode mounted axially within the chamber and consisting of a rod having a plurality of radially extending projections and a second cylindrical electrode surrounding the first electrode wherein the second electrode is mounted inside the reactor chamber characterised in that said second electrode has inwardly extending projections facing those of the first electrode. The radially extending projections may take the form of a sharp-edged helix formed on the surface of the rod in the manner of a screw thread. Alternatively, the radially-extending projections can be in the form of parallel fins either machined in the surface of the rod, or the central rod can be fabricated from a stack of disks and spacers of smaller diameter. In a further variant, the spaces between radially-extending projections is filled with a ceramic or other high dielectric constant material having the same diameter as the radially extending projections. In this case, the corona discharge streamers will form at the outer edges of the interfaces between the ceramic and metal portions of the central electrode. The invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a longitudinal section of an embodiment of the invention, Figure 2 is a longitudinal section of a portion of another form of central electrode embodying the invention, and Figure 3 is a longitudinal section of another embodiment of the invention. Referring to Figure 1, a corona discharge reactor consists of a cylindrical reactor chamber 1 formed by a borosilicate glass cylinder 2 with upper and lower extensions 3 and 4, respectively. Situated between the cylinder 2 and the upper and lower extensions 3 and 4 are two metal spiders 5 and 6, respectively. The assembly is clamped together by means of annular collars 7 which seat on tapers 8 formed at the ends of the cylinder 2, studs 9 and nuts 10, as is common practice in the chemical art. The spiders 5 and 6 have inwardly-facing central recesses 11 which locate a central electrode 12. The electrode 12 is in the form of a metal rod the surface of a central portion 13 of which has its surface machined to provide a sharp-edged helical projection 14 standing approximately 0.5 cm above the overall surface of the central portion 13 of the electrode 12 with a pitch of 1 cm. To ensure mechanical stability as well as to allow for variations in the dimensions of the apparatus arising from thermal expansion of the apparatus in use, the electrode 12 is in two telescopic portions 15 and 16, which are urged against the spiders 5 and 6 by a spring 17. The ends of the central portion 13 of the portion 15 of the electrode 12 are tapered to prevent arcing over. The central portion 13 of the electrode 12 is surrounded by an outer electrode 18 in the form of a metal helix having the same radial extension and pitch as the helical projection 14 on the central portion 13 of the electrode 12. The separation between the edges of the helices 14 and 18 is 3 cm. The spider 5 is provided with a high voltage terminal 19, and the spider 6 is provided with a terminal 19' by means of which it can be grounded. The collars 7, studs 9 and nuts 10 associated with the spider 5 are covered with a shield (not shown) to prevent discharges from their edges. Alternatively they can be made of an insulating material such as that known as TUFNOL (Trade Mark). The upper and lower extensions 3 and 4 are attached to respective end-caps 20 and 21 by a similar arrangement of tapers, collars and nuts and bolts as are the cylinder 2 and upper end lower extensions 3 and 4. The same reference numerals as before are used. The end caps 20 and 21 have associated axial inlet and outlet ports 22 and 23, respectively. Instead of the electrode 12 having a single helical projection, it can have a series of parallel fins. In this case, as in the embodiment previously described the electrode 12 can be a single piece of metal machined to the desired configuration, or it can be assembled from a stack of disks and spacers. Another form of central electrode has the space between successive turns of the helical projection 14 or separate fins, filled with a ceramic 32 having the same external diameter as the helical projection or fins, as is shown in Figure 2. In this case, the corona emission takes place at the exposed edges 33 of the interfaces 34 between the metal projection 14 or fins and ceramic portions 32. Other forms of high dielectric constant, or even semiconducting, material can be used instead of the ceramic 32. Also, the fins can project slightly beyond the in-filling material. The residence time of the gaseous medium in the reactor and the efficiency of the interaction between the gaseous medium and the streamers of the corona discharge can be increased by imparting a tangential component to the flow of the gaseous medium as it passes through the reactor. This can be done by means of a rotating paddle in the extension 4 which acts as a plasma chamber to the reactor main chamber 1, suitably positioned vanes in the extension 4 or in the gaps in the spider 6, or, as shown in Figure 3, by replacing the axial inlet 23 with a tangential inlet 42. Another way in which the efficiency of the interaction between the streamers of the corona discharge and the gaseous medium can be improved is to increase the length of the corona discharge streamers. This can be done by providing an axial magnetic field in that region of the reactor chamber 1 in which the corona discharge occurs. The effect of the magnetic field is to bend the corona discharge streamers in the planes in which they occur, so increasing their length. The magnetic field can be provided by a solenoid such as that shown in Figure 3 with the reference numeral 41, or by an array of fixed magnets. By the use of modem cermet magnets, relatively high magnetic fields, of the order of 1 Tesla at the wall of the reactor chamber 1 can be achieved readily.

Claims: A corona discharge reactor for use in the processing of materials, comprising a reactor chamber (1) through which a gaseous medium to be processed can be caused to flow, a first cylindrical electrode (12) mounted axially within the chamber (1) and consisting of a rod (13, 15, 16) having a plurality of radially extending projections and a second cylindrical electrode (18) surrounding the first electrode (12), wherein the second electrode (18) is mounted inside the reactor chamber (1) characterised in that said second electrode (18) has inwardly extending projections facing those of the first electrode (12). A corona discharge reactor according to Claim 1 wherein the second electrode (18) comprises an helical coil. A corona discharge electrode according to Claim 2 wherein the pitch of the helical coil (18) equals the axial separation between the projections on the first electrode (12). A corona discharge reactor according to Claim 1 or Claim 2 or Claim 3 wherein there is provided means (42) for imparting a tangential component to the flow of the gaseous medium through the chamber (1). A corona discharge reactor according to any of Claims 1 to 4 wherein there is provided means for causing corona discharge streamers formed in the gaseous medium to follow curved paths through the gaseous medium thereby to increase the length of the corona discharge streamers. A corona discharge reactor according to Claim 5 wherein the means for causing the corona discharge streamers to follow curved paths through the gaseous medium comprises means (41) for establishing an axial magnetic field within the reactor chamber (1). A corona discharge reactor according to Claim 6 wherein the means (41) for providing an axial magnetic field within the reactor chamber (1) is capable of producing a magnetic field of the order of a Tesla within the reactor chamber (1). A corona discharge reactor according to any preceding claim wherein there is included between successive radial projections (14) on the first electrode (12) a material (32) having a high dielectric constant. A corona discharge reactor according to Claim 8 wherein the material (32) between successive radial projections (14) on the first electrode (12) is a ceramic material having a high dielectric constant. A corona discharge reactor according to Claim 8 wherein the material (32) between successive radial projections (14) on the first electrode (12) is a semiconducting material. A corona discharge reactor according to any of Claims 8 to 10 wherein the material (32) between the projections (14) on the first electrode (12) has an external diameter approximately equal to that of the projections (14) on the electrode (12). A corona discharge reactor according to any preceding claim wherein the first electrode (12) comprises a central portion (13) having the projections (14), a first smooth portion (15) and a second smooth-surfaced portion (16), the central portion (13) being adapted to allow one of the smooth portions (15, 16) to move telescopically within it under the action of a compression spring (17).

Inventors: BAYLISS KEITH HOWARD, CARLOW JOHN SYDNEY, BAYLISS, KEITH HOWARD, CARLOW, JOHN SYDNEY
Applicants: AEA TECHNOLOGY PLC
Application year: 1994
Main classification: H01T19
IPC codes: H01T19, B01J19